What is Dot Gain: Causes, Effects, and Solutions in Flexo Printing

The highly specific area of flexographic printing calls for dot gain control not only for technical perfection but as well as for customer satisfaction. Dot gain affects how ink is deposited on the substrate and can significantly alter the visual outcome of printed materials. This could lead to replacements, increased costs, and even loss of clients. Thus, it’s imperative for printers to deeply understand the causes of dot gain and to master the techniques to mitigate it. This guarantees that the prints will be delivered solely in the best quality possible, and will maintain the high standards of quality and consistency across all the projects.

What is Dot Gain?

Dot gain, also known as tone value increase (TVI), is a printing defect that occurs when the size of the halftone dots on the printed substrate is larger than the corresponding dots on the printing plate. It is a phenomenon that occurs in various printing processes, including offset printing and flexography. In flexo printing, the images are reproduced by a number of tiny dots of different sizes and it looks like continuous tones are used. When the dot gain occurs, these dots go beyond their set size and they become more dark and less detailed.

dot gain is measured as a percentage and may differ depending on the substrate, ink, and printing conditions. For instance, a 20% dot on the printing plate can be 30% on the substrate so a 10% dot gain is expected. The total dot gain is the sum of physical dot gain (the actual increase in dot size) and optical dot gain (the perceived increase due to light absorption and scattering).

Effects of Dot Gain on Print Quality

Dot gain, as a defect of the printing process, could have many negative effects on the quality of the final print product, especially in the flexographic printing where color consistency and accuracy are crucial. Some of the key effects of dot gain include:

Darker prints	As the halftone dots expand, the print gets darker than intended, shifting the color balance and resulting in a loss of detail in the shaded areas.
Reduced contrast	Dot gain can lead to a decrease in contrast between light and dark areas, making the image appear flat and less vibrant.
Loss of detail	Dots spread out beyond intended, the fine details and the texture in the image gets lost and the final print looks fuzzy and less crisp.
Color shifts	Dot gain can affect different colors to varying degrees, leading to colour shifts and inconsistencies in the final print.
Inconsistent results	If dot gain is not properly controlled and compensated for, it can lead to inconsistent print quality across different jobs or even within the same print run.

Causes of Dot Gain in Flexo Printing

Here are the main causes of dot gain in flexo printing:

• excessive printing pressure
• incorrect ink viscosity
• improper anilox roll selection
• plate material and durometer issues
• printing plate thickness variations
• contamination on plate or impression roller surface
• mounting tape problems
• excessive printing speed
• improper ink fountain and pump settings
• blanket characteristics and condition
• substrate properties
• environmental factors in the print room

Excessive Printing Pressure

Flexo printing is a pressure-sensitive process, where the raised image areas of the printing plate are in direct contact with the substrate to imprint the ink. If the pressure between the plate and the substrate is higher than the appropriate value, the ink dots on the plate will be deformed and spread, thus the printed substrate will have larger dots. This is especially exacerbated when using softer printing plates or when printing on substrates that are more compressible.

The printers must be very accurate in maintaining the space between the printing plate and the substrate to be able to control the pressure during the ink transfer. This may include a trial and error which can be supplemented by the use of pressure-sensitive tapes or gauges to observe and regulate the pressure during the printing process. One solution should be by using kiss impression, where the plate does not make contact with the substrate surface. Modern flexographic printing presses frequently have an automated pressure control system that uses sensors to detect the pressure and adjust it in a consistent manner for the whole print run.

Incorrect Ink Viscosity

The viscosity of the ink used in flexography printing can also be a key factor that causes dot gain. If the ink is too thin or has low viscosity, it can quickly spread beyond the limits of the selected dot size, resulting in the increase of dot gain. Contrary to this, thicker and higher viscosity inks may not transfer properly on the substrate, which may result in inconsistent coverage and a loss of color density.

The viscosity of the ink can be varied by adding solvents or thinners, but it must be done according to the manufacturer’s instructions and throughout the print run.

Improper Anilox Roll Selection

Anilox rolls are a critical component in the flexographic printing process, responsible for metering and transferring the ink to the printing plate. If the anilox roll has a higher line screen or cell volume than necessary, it can transfer excessive ink to the plate, leading to increased dot gain.

To mitigate dot gain caused by improper anilox roll selection, it is essential to choose the right anilox roll specifications for the specific job and substrate. This involves considering factors such as the desired ink coverage, line screen, and cell volume.

Plate Material and Durometer Issues

The type of printing plate material and its durometer (hardness) can significantly impact dot gain in flexo printing. Softer plate materials, such as photopolymer plates, can be more susceptible to dot gain due to their increased compressibility under pressure. This can lead to the dots deforming and spreading, resulting in an increase in dot size on the printed substrate.

To address such issues, you can consider harder plate materials, such as EPDM or laser-engraved elastomeric plates, which can offer better resistance to dot gain, particularly for high-quality process color printing. Working with your plate supplier to choose the right plate material and durometer can help minimize dot gain and improve print consistency.

Printing Plate Thickness Variations

Inconsistencies in printing plate thickness can also contribute to dot gain issues. If the plate thickness varies across the surface, it can result in uneven pressure distribution during printing. Areas with thicker plate material may experience higher pressure, leading to increased dot gain, while thinner areas may have insufficient pressure, resulting in incomplete ink transfer.

Using high-quality plates with uniform thickness and ensuring they are stored and handled correctly will minimize this issue. Conduct regular checks for uniformity as part of your pre-press setup

Contamination on Plate or Impression Roller Surface

Contamination on the surface of the printing plate or impression roller can also lead to dot gain problems. Debris, dust, or dried ink buildup on these surfaces can create high spots or imperfections that cause localized increases in pressure during printing. These high spots can lead to increased dot gain in the affected areas, resulting in inconsistent print quality. Regular cleaning and maintenance are necessary to maintain quality.

Mounting Tape Problems

The mounting tape used to attach the printing plate to the print cylinder can also impact dot gain. Generally, tapes with a thickness range of 0.015 to 0.020 inches (0.38 to 0.51 mm) are commonly used in flexographic printing. If the mounting tape is too soft or has inconsistent thickness, it can allow the plate to compress excessively under pressure, leading to increased dot gain. Additionally, if the mounting tape is not applied properly or has air bubbles, it can create uneven pressure distribution and contribute to dot gain issues.

Printers should select high-quality mounting tapes that are compatible with their flexographic printing plates and ensure that the tape is applied smoothly and consistently. Using automated mounting systems or specialized tools can help achieve precise and bubble-free tape application, minimizing the risk of dot gain problems related to mounting tape issues.

Excessive Printing Speed

Running the flexographic press at excessively high speeds can also contribute to dot gain. When the printing speed is too fast, it can cause the ink to transfer and spread more readily, as there is less time for the ink to settle and adhere to the substrate. This can result in increased dot gain and a loss of print quality.

To control dot gain related to printing speed, printers should determine the optimal speed for each job based on factors such as the substrate, ink type, and image complexity. Running the press at a slower speed may be necessary to achieve the desired print quality and minimize dot gain, especially for more challenging or critical jobs.

Improper Ink Fountain and Pump Settings

Incorrect settings on the ink fountain and pump system can also lead to dot gain issues in flexographic printing. Flexographic inks are typically lower in viscosity compared to offset printing inks, which allows them to flow more easily and spread on the substrate. If the ink flow is too high or the ink fountain is overfilled, it can cause excess ink to be delivered to the printing plate, resulting in increased dot gain. On the other hand, if the ink flow is too low, it can lead to insufficient ink coverage and a loss of color density.

Printers should carefully adjust the ink fountain and pump settings to maintain consistent and appropriate ink delivery throughout the print run. This may involve monitoring ink levels, adjusting ink feed rates, and ensuring that the ink is properly circulated and mixed to maintain consistent viscosity and color properties.

Blanket Characteristics and Condition

In flexographic printing, the condition and characteristics of the impression cylinder blanket can also impact dot gain. A worn or damaged blanket surface can create uneven pressure distribution, leading to increased dot gain in certain areas. Additionally, if the blanket is too soft or has a high level of compressibility, it can allow for excessive dot spread under pressure.

Printers should regularly inspect and maintain the condition of their impression cylinder blankets, including checking for signs of wear, damage, or contamination and replacing the blankets as needed.

Substrate Properties

Different substrates have different levels of absorptivity, roughness of the surface, and compressibility. All of these characteristics influence how the ink dots spread and interact with the substrate surface. For instance, uncoated papers are typically more absorbent and have a rougher surface that can result in the higher dot gain because the ink penetrates and spreads more extensively. In addition, the papers coated have a smoother surface and absorb less ink, which results in a lower dot gain. Similarly, films and foils can have different surface features which may affect dot gain.

Testing and characterizing the dot gain behavior on different substrates can help printers make informed decisions and adjust their processes accordingly. In some cases, pre-treatment or priming of the substrate may be necessary to achieve the desired print quality and minimize dot gain.

Environmental Factors in the Print Room

Temperature and humidity in the print room can have an effect on dot gain. The moisture content of the substrate may vary with humidity levels, which in turn may result in a change of dimensions and surface properties. This can change the way the ink interacts with the surface and possibly affect adhesion and dot shape. Also, extreme heat can change the ink viscosity as well as drying times which in turn could worsen the dot gain problem.

The solutions can be adopting the use of air conditioning, dehumidifiers, or humidifiers to control the temperature and humidity levels. Observing and recording the environment during printing can help spot out any problems and make the necessary corrections to keep the quality of the print stable.

Measuring and Quantifying Dot Gain

Accurate measurement and quantification of dot gain, the phenomenon that significantly affects the printing quality, is a crucial step to achieve precise control and compensation. There are several methods for measuring dot gain, including:

• Densitometry: This method involves measuring the optical density of the printed dots using a densitometer. By comparing the density of the dots on the printing plate to the density of the dots on the printed substrate, the dot gain can be calculated.
• Spectrophotometry: Spectrophotometers measure the spectral reflectance of the printed dots, providing a more accurate assessment of color and dot gain compared to densitometry.
• Microscopy: By using a microscope to examine the printed dots, the physical dot gain can be directly observed and measured.
• Tone value increase (TVI) curves: TVI curves plot the relationship between the dot percentages on the printing plate and the corresponding dot percentages on the printed substrate. These curves help visualize and quantify the extent of dot gain across different tonal values.

Conclusion

The control of dot gain is fundamental to high-quality flexographic prints and the operator should, therefore, have good knowledge about it. Through proper equipment selection, exact setup and stringent process control, printers can significantly cut down on dot gain which in turn will result in better quality final products. It doesn’t matter whether you are troubleshooting the existing problems or setting up a new printing operation, the full understanding of dot gain dynamics will give you the assurance that your final prints meet the highest standards of the most demanding customers.

Get a Free Quote With KETE

As a leading manufacturer of flexographic printing machines, KETE understands the critical importance of precision and quality in every aspect of printing. If you’re facing challenges with dot gain or simply looking to upgrade to state-of-the-art flexo printing technology, KETE is here to help. Contact us for a free consultation and quote, and let our experts assist you in selecting the perfect printing solution tailored to your needs. Enhance your printing capabilities with KETE, where technology meets craftsmanship to deliver exceptional print quality.